I'm not quite buying he has no limit to the amount of energy he can absorb. I can quote other issues where he states he's fully sated like when he prepares to take on Thanos with the Infinity Gems.

Typically Galactus just consumes 1 world because its the bare minimum to keep him functioning at acceptable levels. He can and has taken in more.

One world does a lot more than keep him functioning at acceptable levels. It boosted his power to the point he was fully restored and confident enough to challenge Thanos Wielder of the Infinty Gems as shown in Surfer #51 per my scan.

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If He has no limit then why does he require energy reserves on his ship to sustain him in dire circumstances?

He doesn't require the ship to have a large amount of energy but rather keeps it around. Just like you would keep a emergency can of food around. Galactus doesn't have the convenience of going to a grocery store and pick up what he needs. Suitable planets for Galactus to consume are hard to come by and it may be some time between finding one he he doesn't have to fight over.

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Agree, but we don't know how much energy his ship stores or how essential those energy supplies have been to his survival.

You didn't cite me the issues earlier to support your point?

See the very first Secret Wars with the Beyonder. Galactus used the world ship (TAA 2) as an emergency energy source (he didn't have time to go look for a world). I have read this over the years and looking at numerous issues this is how Galactus is portrayed. In the Jonathan Hickman Fantastic Four series Galactus stated to Read Richards that he consumed 4 worlds in anticipation of what was to come (fighting mad Celestials). The opposite of this is in play as well. Under John Bryne Galactus actually shrank in height because he let his power dwindle way too low. (FF #243 I believe). This sort of explains Galactus' variation in height. The more he consumes the bigger he gets, Under Walt Simonson (FF #340) Galactus started consuming the entire universe and he had grown to such enormous proportions that people were tiny specs to him. Galactus was so big that Reed and his team didn't even see him. Galactus was near planet size and getting bigger.